I have a strange setup where the eth0 is connected to a network that has no internet access or NTP server. I also have a wifi adapter that connects to a network that does have access to an NTP server.
Timesync (I'm using Stretch) seems to priorities eth0 when it looks for the NTP server (I am not sure of this, but that's what seems to be happening). When I have both connected it doesn't see the NTP serve but when I dissconnect the eth0 cable and leave it unplugged for a little while the Pi then gets it's NTP sync.
Is there any way to get Timesync to 'default' to wlan?